1. Field of the Invention
The present invention relates to a telecommunication system. The present invention is particularly, but not exclusively, concerned with a telecommunications system for mobile telephones.
2. Summary of the Prior Art
In standard land-based telecommunications systems, calls are routed from one fixed point to another, and it is therefore relatively straightforward to determine a switching path for such calls. The originator of the call inputs information to the system (normally the telephone number of the destination) in which the routing information is arranged sequentially. Thus, it is relatively straightforward for the system to determine the switching necessary to achieve the routing, by successively decoding parts of the input information. Any switching point need consider only part of the information.
When the telecommunication system involves mobile telephones, the situation is more complicated because a call to a mobile telephone is not to a fixed point, and therefore the system must determine the location of the destination. Currently, a call to a mobile telephone results in a signal being transmitted to a data storage unit in the form of a Home Location Register unit (HLR) which determines the location of the mobile telephone, and so permits routing of the call to occur.
Inevitably, HLRs have a limited capacity, and some arrangement is therefore necessary to enable telecommunication systems to access multiple HLRs. At first sight, all that is needed is for a plurality of HLRs to be provided, and different mobile telephone numbers assigned to different HLRs.
However, this is not consistent with the aim of providing customer flexibility in a mobile telephone system. Each mobile telephone has two types of number associated with it, one being the number which third parties use to call the mobile telephone (a "MSISDN"), and the other being an identity number (an "IMSI") which is used by the telecommunications system to address the mobile telephone. Using existing techniques, information relating to both of these numbers must be stored on the same HLR.
It is envisaged that users may need multiple MSISDN numbers, for example if a user is to have the possibility of both voice and data communication, in existing systems, any second MSISDN number with a common identity number (IMSI) must be a MSISDN number of the same HLR as the previous MSISDN number. This could be impossible to achieve if, for example, the HLR containing the original information is full. Then the only way that additional services could be provided would require the user to change telephone number, which would be undesirable. This becomes a particular problem if it is desirable that users are able to select their numbers, rather than be provided with them.